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Master thesis: Automated electron holography

Advertising institute: ER-C-1 - Physics of Nanoscale Systems
Reference number: D033/2018, Physics

Off-axis electron holography is a technique carried out in a transmission electron microscope (TEM), which is used to map magnetic and electric fields. The figure on the right shows for example the magnetic field lines in three nanocrystals of magnetite obtained using electron holography. The basic principle is to use an electrostatic biprism (charged wire), placed below the sample, to overlap two parts of the electron beam (see figure below). Usually, one part of the beam goes through the object (object wave), whereas the other part travels in the vacuum (reference wave). An interference pattern (hologram) is then registered on a camera. This interference pattern contains both the amplitude and, most importantly, the phase information of the electron wave, which is normally not available in standard TEM. After reconstruction of the phase information by Fourier processing, electromagnetic fields can be mapped with a spatial resolution of a few nanometers. However, the signal-to-noise in the reconstructed map is not always sufficient to measure small phase variations, in particular in the case of thick samples. The signal-to-noise can be improved by taking long exposures (increasing the number of counts) which, in practice, is limited by the stability of the microscope (drift of the specimen and the biprism) and usually cannot exceed a few seconds. In this case, one solution is to acquire multiple holograms with short exposure times, then realign the images by post-processing before summing the results. Here, the main goal will be to develop scripts to automatically acquire, save, align, process multiple object and reference holograms, and display the cumulated phase (and derivatives of the phase) shortly after the acquisition at the microscope. For many users of the microscope, such scripts could significantly improve the efficiency of the experiments and the quality of the results.

In a first step, the student will become familiar with the interface and the scripting language of Digital Micrograph, the software which is used to control the camera of the microscope. Second, the student will write a script to process and align multiple holograms. Third, the script will be extended to control the camera and the parameters related to the acquisition (exposure time, binning, creation of image stacks). At this stage, training of the student to use the Holo Titan TEM of the Ernst-Ruska Center will be possible. Fourth, the script will be extended further to control the microscope, for instance to move the specimen automatically and acquire reference holograms. Depending on the progress, different features could be implemented such as: automatic focusing, automatic adjustment of the stigmators, drift compensation of the sample and the biprism, correction of lens distortions. The student will be involved in an international group where electron holography is used in different fields of material science (ferromagnetism, ferroelectricity, semiconductors, 2D materials, tomography).

Contact persons:
Thibaud Denneulin;
Vadim Migunov
Forschungszentrum Jülich
Ernst Ruska-Centre for Electron microscopy